This invention relates to sensing of the current in a load circuit and, more particularly, to sensing the current in a motor control drive circuit.
A conventional motor control circuit receives AC input power and converts this AC power to DC power, which is then applied to a pulse width modulated (PWM) inverter. The inverter responds to gate drive signals to the transistors comprising the inverter to thereby develop the necessary motor drive signals.
Current sensing is an essential part of the motor drive system. Traditionally, current sensing has been implemented in the motor drive system as an independent function and circuit apart from the gate drive function.
More specifically, the current sensing function for motor drive application has traditionally been implemented using either a magnetic Hall Effect device or an optically isolated device with a shunt resistor. These traditional current sensing devices are physically independent components, and contain high voltage isolation or level shifting circuits inside of sensors in order to provide low level signal output. They are normally interfaced with an analog-to-digital converter for digitizing the analog signal into a digital control system, such as an DSP (Digital Signal Processor) or a microcontroller.
These prior art devices have the following limitations and cost disadvantage when used in a motor drive control system:
1) Integration limitation in an integrated circuit due to the requirement for discrete implementation of the magnetic component or optical isolation device;
2) The need to provide current sensing synchronization with PWM switching; and
3) The need to provide integration of overcurrent protection function.
An object of the invention is to sense motor current using a low side shunt resistor. One of the problems in doing so is that the waveform is incomplete and comprised of a plurality of saw tooth pulses. Accordingly, a further object of the invention to reconstruct a sinewave signal from such pulses.
In accordance with the present invention, this is done by providing a circuit which includes a sample and hold circuit for receiving the plurality of saw-tooth pulses and means for supplying a timing signal to the sample and hold circuit such that the plurality of saw-tooth pulses is converted to a sinusoidal signal at the output of the sample and hold circuit.
Another object of the invention is to synchronize current sensing with the gate drive signal applied to drive a power transistor.
In accordance with the present invention, this is accomplished by a circuit for driving a load and sensing load current, which includes at least one power transistor connected to the load, a gate drive unit for applying a gate drive signal to a gate of the transistor to turn the transistor on and off, and a current sensing device connected to the load for generating a signal proportional to current in the load, the signal being comprised of a plurality of pulses. A sample and hold circuit is provided for sampling and holding each of the pulses at predetermined times, the timing of the sample and hold circuit being controlled by the gate drive unit.
Preferably, the load is a three phase AC motor driven by a plurality of power transistors. Respective gate drive units, current sensing devices and sample and hold circuits are provided for each of the phases
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.